1. Field of the Invention
This invention relates to the field of heat pump systems and in particular to ground source heat pump systems. The invention provides a system having modular in-ground heat exchangers suitable for simple installation in a trench, and particular compressor and circulation features facilitating installation and operation of a heat pump system for heat transfer to and from the earth.
2. Prior Art.
Heat pump systems are increasingly popular for efficient heating and cooling of loads, for example as part of a heating-ventilation-air conditioning (HVAC) system for buildings. Heat pump systems generally include heat exchangers thermally coupled to the load and to a heat source or heat sink, the heat exchangers being connected in a refrigerant or coolant loop which includes a compressor and an expander. The compressor raises the pressure (and therefore the temperature) of the refrigerant and the expander lowers the pressure, producing a lower temperature in the refrigerant gas.
In heating a load, a "ground source" heat pump, which has the source/sink heat exchanger thermally conductively coupled to the ground, can extract a virtually limitless supply of thermal energy from the earth and transfer the energy, at higher temperature, to the load. A heat pump cools a load by extracting thermal energy from the load and transferring it, at higher temperature, to the earth for dissipation therein. In this manner the ground functions as either a heat sink or heat source. Modern day heat pumps for HVAC systems are equipped with reversing features such as valves to arrange the flow of refrigerant so that they may both heat and cool the load, as needed.
A ground source heat pump requires a subterranean heat exchanger. While it is possible to use intermediate heat exchangers for transferring heat through thermally coupled fluid flow paths or the like, preferably the refrigerant or coolant is pumped through the pipes by the compressor and serves directly as the carrier for conveying the thermal energy to or from the ground. Thus, extra heat transfer losses, such as those inherent in ground water source systems, are avoided. The coolant is relatively heated by compression and cooled by expansion, leading to the respective heat exchangers, thereby raising the temperature of the hot side heat exchanger above the temperature of the load and lowering the temperature of the cool side heat exchanger below the temperature of the source, whereupon heat transfer occurs. Compression and expansion normally include a change of state of the coolant between liquid and gaseous states.
The load heat exchanger is typically above ground and the ground heat exchanger is preferably well below the surface of the ground. Connecting pipes for the ground heat exchanger, and the pipes defined by the heat exchanger itself, can be horizontal, vertical or slanted. A typical installation may include combinations of these orientations, depending upon particular design criteria. A number of potential problems, however, are encountered with each possible orientation of the connecting pipes as well as pipes included in the heat exchanger itself. For example, vertically oriented pipe arrangements, which might define a buried U-bend, are expensive and difficult to install, in part due to the necessity of forming deep vertical holes in the ground. Equipment to accomplish such boring is typically complex and expensive. Such holes have a tendency to cave-in during or after the excavation process. Additionally, accomplishing oil return to the compressor is difficult, especially when the system is designed to operate in both the heating and cooling mode. On the other hand, vertically elongated arrangements define less of an obstruction to earth which is replaced after installation, which (at least at the heat exchanger) must be placed in intimate contact with the refrigerant carrying means in order to achieve good thermal coupling with the earth.
A problem with vertical extensions is the tendency for gas to rise upward through liquid. For example, if a segment of gas were disposed at the buried U-bend of a vertically oriented pipe, it would tend to bubble up through the liquid without pushing the liquid on through the system. The gas would pass through the liquid unable to push the liquid through the coils. The liquid, as well as intermixed refrigerant and oil/lubricant, would thus settle at the bottom of the coils, reducing efficiency and/or resulting in compressor failure.
Arrangements which encompass a substantial horizontal area, for example including horizontal or slanted in-ground pipes, require a large land area for installation. Whereas the array of pipes defining a heat exchanger must be buried deeply, the installer may have to remove a huge quantity of earth to place the pipes, and then must replace the earth over the pipes. The problem can be daunting when using multiple loops in a horizontally oriented pattern. Further, the use of horizontal pipes typically result in refrigerant pipe cross-overs, which may reduce efficiency. Existing gas and water lines further complicate the installation.
The choice of horizontal, vertical and slanted pipe runs is constrained in known ground source systems by operational complications in addition to installation problems, especially in conjunction with the reciprocating piston type compressor which conventionally drives the refrigerant flow in known heat pump systems. Inasmuch as the ground heat exchanger must be buried, at least some vertically oriented pipe runs are almost always needed. A notable problem is encountered in that when an energy demand cycle is completed, the compressor which drives the flow of refrigerant shuts down pending a subsequent demand for energy transfer. As a result, a certain amount of refrigerant then passing through the subterranean pipes looses its momentum and remains at low points in the pipes where it cools and may condense. The compressor is generally designed for efficient pumping of refrigerant in the gaseous state as opposed to liquid. When the compressor come on after an off cycle it may quickly deplete the gaseous refrigerant upstream of the compressor along the flow path, such that a low pressure condition is created at the input of the compressor. Gaseous refrigerant in the circuit may also be trapped between quantities of liquid refrigerant even in a horizontal arrangement. Moreover, the refrigerant has a certain inertia, particularly in the liquid phase. For these reasons, the compressor may encounter substantial loading problems when starting up after an off cycle.
Heat pump systems typically include control features designed to prevent compressor damage due to overloading. Most systems are designed to interpret a low pressure condition at the compressor input as an indication that insufficient refrigerant exists in the system to function properly. As a result, the compressor is automatically shut down when a low pressure condition is sensed in order to protect against failure of the compressor due to absence of sufficient refrigerant.
A pressure problem is typically encountered with a reciprocating compressor during regular on/off operational cycles. To allow pressures to equalize during such periods, the prior art, such as U.S. Pat. No. 5,025,634 - Dressler, resorts to the use of a bleeder hole in a pressure valve or wall. This may result in a slight system efficiency loss under certain operating conditions.
A similar but more pronounced low pressure problem is encountered when a reversible system changes from a heating mode to a cooling mode. Such a change inherently causes an imbalance in refrigerant capacity after reversing. This imbalance results from the much larger volume capacity of the subterranean heat exchanger as compared to the volume capacity of the load heat exchanger.
When the operating cycle is reversed, additional time must be allowed to manipulate the excess refrigerant whereby the refrigerant can assume its appropriate redistribution throughout the system in order to properly function in the reverse mode. During this redistribution period, a low pressure condition is created at the input of the compressor. The relatively short time interval allowed for the low pressure condition at the compressor input, before shutdown to protect the compressor, can be insufficient for a typical reciprocating piston type compressor to overcome the inertial resistance of stagnant refrigerant in the subterranean pipes and to redistribute the refrigerant for operation in the reverse mode. As previously described, a continued low pressure condition at the compressor input causes the compressor to automatically prematurely shut down.
A further problem with reciprocating piston compressors in ground source heat pump systems results from the typical fact that the compressor lubricant mixes and flows with the refrigerant. The compressor lubricant can, consequently, in the ground coils, ultimately accumulate resulting in compressor lubricant loss and failure.
Prior art attempts to circumvent the aforesaid problems, including problems related to pressure, include utilizing a bleeder hole in a valve/wall, and/or altering the heat pump design to orient the flow paths such that low pressure conditions and obstructions are less likely to occur, or cause fewer problems when they do occur. An example is to use a plurality of thermally coupled closed loop fluid circuits working in combination. A horizontally oriented refrigerant loop (i.e., with the compressor and expander), for example, can be thermally coupled to a vertically disposed subterranean loop which simply circulates a heat exchange fluid through the heat exchanger. These solutions, besides being complex and inefficient with respect to heat transfer performance as well as installation as discussed above, tend to create new problems at least as serious as those remedied.
An example of a reversing cycle heating system for a building comprising a heat pump and heat exchanging tubes is disclosed by U.S. Pat. No. 4,688,717 - Jungwirth. In Jungwirth, a central distributor communicates with a plurality of downwardly inclined heat exchanging tubes. Jungwirth emphasizes a multitude of buried refrigeration loops for optimum heat exchange with the earth. The plurality of buried refrigeration loops in Jungwirth necessarily complicates the excavating process. It is necessary to bore a plurality of holes radially outward from a central excavation site or to excavate and later refill a substantial volume of earth. Further, this design solely operates in a heating mode.
German Offenlegungsschrift 35 14 191 - Waterkotte (Oct. 23, 1986) discloses a series of looped lines extending to and from a central manifold, specifically placed to facilitate oil return to the compressor.
U.S. Pat. No. 4,383,419 Bottum shows a heat pump heating system having a series of slightly slanted tubes buried horizontally underground or located horizontally under water. The refrigerant is used as the heat transfer fluid, with one manifold being disposed slightly below the other.
U.S. Pat. No. 4,741,388 - Kuroiwa and U.S. Pat. No. 4,277,946--Bottum depict vertically oriented heat exchangers requiring deep, vertical earth boring. Other designs known in the art such as Dressler include systems with oversized accumulators so as to avoid slugging the compressor with liquid refrigerant, pressure equalization bleeder valves, storage and recycling devices, self-adjusting refrigerant flow cooling valves and flow reversing valves. These special provisions are intended to remedy the above-mentioned coolant imbalance and pressure problems as well as the problem of lubricant accumulation within the heat exchange tubes or connecting conduits.
It would be advantageous to resolve the foregoing problems without resort to various complications that each involve additional cost, installation steps, maintenance and operational limitations. A heat pump based heating/cooling system is therefore needed which is devoid of the inherent limitations and complications associated with present day systems of this nature, but is at the same time robust and efficient. In particular, a system not subject to pressure imbalances or deficiencies, and which is insensitive to accumulation of lubricating oil is needed. Preferably the system should have straightforward refrigerant and thermal flow paths involving a minimum of elements. It is further desirable to provide such a ground source heat pump heating/cooling system wherein the ground source heat exchanger or heat exchange tubing does not require disruption of a large area of land or the forming and back filling of complex holes.